<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .a {
            width: 1200px;
            margin: auto;
            height: 100px;
            background-color: #f00;
        }

        .a1 {
            width: 1200px;
            height: 50px;
            /* background-color: #ddd; */
            margin-left: 300px;
        }
    </style>
</head>

<body>

    <!-- 
        1 截图的   不要拍照
        2 截图截调试工具 ！！！！！！
        3 不要超前知识点
        4 网页不要出现滚动条    delete
        5 调试工具看结构  ---  哪里有问题点那里
        6 浮动的bug????
     -->

    <div class="a">
        <div class="a1"></div>
    </div>

</body>

</html>